Optical modulation, such as optical differential quadrature phase-shift-keying (DQPSK) and polarization division multiplex optical quadrature phase-shift-keying (PDM-QPSK), enables spectrally-efficient communication in the optical domain. Such modulated signals may be transmitted by a transmitter, propagated through an optical waveguide, and demodulated (or decoded) using a receiver.
FIG. 1 illustrates the functionality of a receiver 100 for demodulating a DQPSK signal, as is conventionally known. An incoming optical carrier signal 102 encoded with DQPSK modulated data is approximately equally split at 104 into two data signals, one of which incurs a delay at 106 approximately equal to a symbol period of the modulated signal. The relative phase of the optical carrier in the two optical data signals is adjusted by use of phase control electrodes 110 and optionally the auxiliary control electrodes 108. These two data signals are recombined in the 90° optical hybrid mixer 112 (hybrid-90). The hybrid-90 112 functions to combine the two signal inputs into four signal outputs. Each output combines equal proportions of the input signal, but has different relative phases of 0°, 90°, 180°, and 270°, respectively, but not necessarily in this order. The two outputs with relative phase-difference of 0° and 180° carry the encoded in-phase data (the I-channel), and the two outputs with relative phase-difference of 90° and 270° carry the encoded quadrature-phase data (the Q-channel). The four optical output signals of the hybrid-90 112 are successively converted into electrical currents and then into amplified voltage signals by use of a combination of waveguide photodiodes 114 and transimpedance amplifiers 116 (TIAs). The optical signals for each of the I- and Q-channels are detected differentially. This can be done either with two balanced pairs of photodiodes 114 connected to two TIAs 116, or with four single-ended photodiodes 114 connected to two differential-input TIAs 116.
FIG. 2 illustrates the functionality of a receiver 200 for demodulating a PDM-QPSK signal, as is conventionally known. Here the incoming optical carrier signal 202 is encoded with two orthogonal polarization states of QPSK modulated data. The incoming optical carrier signal 202 is split at 204 into two separate optical components. One of the two polarization components may be rotated at 206 so as to have into a common single polarization state for comparison with a reference signal from a local oscillator 208 (LO). The detection of the two optical signals performed by the respective hybrid-90s 212, waveguide photodiodes 214, and TIAs 216 is similar to that performed in the DQPSK receiver 100, except that it is the absolute phase state that is modulated, rather than the differential phase between one bit slot and the next. To decode the absolute phase state, the signals are compared with the LO 208 reference signal at the same or similar optical frequency. The same LO 208 signal can be split into two outputs or elements for comparison with the two polarization components of QPSK data at respective hybrid-90s 212, the outputs of which are successively converted into electrical currents and then into amplified voltage signals by use of a combination of photodiodes 214 and TIAs 216. For illustration purposes, FIG. 2 shows all the functions of the PDM-QPSK demodulator receiver. Although, for convenience, each hybrid-90 and its accompanying waveguide photodiodes could be on a separate chip, with the polarization split, polarization rotation, and LO split performed off-chip, for example, in micro-optic components.
As optical communications become faster, the integration of optical components at the chip level has developed rapidly due, in part, to the demand for smaller optical components at reduced costs reduction. For example, a design of an optical DQPSK decoder has been disclosed in U.S. Pat. No. 7,259,901 to Parsons et al. Parsons et al. discloses the use of a 4×4 multi-mode interference filter (MMI) as an optical phase shifter (hybrid-90) together with on-chip optical delay and off-chip detection. This device is preferably realised in silicon photonics.
Similar implementations have been disclosed. For example, silica-on-silicon implementation of the optical hybrid-90 was published by C R Doerr (Lucent Bell Labs) IEEE JLT 24(1) p 171 January 2006 using a star coupler. A monolithic InP DQPSK receiver was published by C R Doerr (Lucent Bell Labs) in a paper “Monolithic InP DQPSK 53.5-Gb/s receiver” at the conference ECOC-07 (September 2007) using a 2×4 star-coupler, thermo-optic and current injection phase shifters, waveguide photodetectors and on-chip optical delay. M Oguma et at (NTT) presented a paper “Single MZI-based 1×4 DQPSK demodulator” at the OFC-2008 meeting (March 2008) using a hybrid-90 design with 2×2 MMI couplers, an on-chip optical delay with insertion of a half-waveplate to counteract impairments due to optical birefringence asymmetries.
However, these conventional chip implementations all have associated disadvantages. They suffer from asymmetric birefringence (or double refraction) and phase control problems. Birefringence can be problematic, especially when the device needs to compare one signal or channel with another for an unknown state of polarization. A difference in phase or apparent path length can affect the device to the extent of rendering it ineffective.
The birefringence of the semiconductor waveguides also makes the optical delay function hard to realize and control on an integrated chip. In addition, the magnitude of the delay (e.g., a 1 symbol delay is typically 20-50 ps) requires a relatively large area of semiconductor material to implement.
For the PDM-QPSK coherent receiver, illustrated in FIG. 2, the requirement is to achieve the hybrid-90 function and balanced detection at a fixed polarization condition, over wavelength and temperature of operation. For both the DQPSK receiver and the PDM-QPSK coherent receiver this drives the need for symmetry of waveguide insertion losses.
Integration of optical components at the chip level demands that there is sufficient volume of production to justify the investment in the chips, and once set up, there is little room for providing customised solutions to customers. The challenge associated with designing a DQPSK decoder is to select and integrate the functional elements such that the performance is maintained over a range of operating wavelengths (for example C-band or L-band), over all states of polarization, over the required case-temperature range, and over the life of the decoder.